The present invention relates generally to circuit breakers and more particularly to circuit breaker operating mechanisms having a handle blocking means for restricting movement of the handle when the current carrying contacts are welded.
Molded case current limiting circuit breakers are well known in the art. Circuit breakers of this type have a manual operating handle for the purpose of switching the circuit breaker between on and off states. The on-off operation is accomplished through a mechanism spring that connects the operating handle with a toggle linkage. The toggle linkage in turn is connected to a contact carrier assembly that performs the operation of connecting and interrupting current flow to a protected circuit.
When the operating handle is moved from the on to the off position, the direction of the force applied by the mechanism spring changes as the spring rotates with the handle. At some point during the motion, the direction of the force changes from one side of a toggle linkage pivot to the other. This results in the toggle linkage collapsing and rotation of the contact carrier assembly.
The circuit breaker generally provides some visual indication as to the position of the contact carrier assembly. However, on extreme and rare occasions the contacts of the circuit breaker can become welded. In this case if the operating handle were allowed to be returned to the off position, it would give the operator the false indication that the protected circuit has been disconnected from the power source. Some regulatory agencies such as the International Electrotechnical Commission (IEC) require that the operating handle be blocked from moving to the off position when the contacts are welded. It is also required by such regulatory agencies that the circuit breaker indicate the position of the contacts. In many circuit breakers when the contacts are welded, the handle automatically returns to the on position. This not only provides correct visual indication of the state of the contacts, but also provides the operator with an indication that there is some malfunction.
A circuit breaker of the type mentioned herein having a mechanism with the toggle type linkage that is described in U.S. Pat. No. 5,200,724. In this circuit breaker the handle movement is blocked by projections extending from both the upper link and the lower link of the toggle linkage. The upper link projection interacts with the handle to block handle rotation while the lower link projection interacts with a crossbar assembly to prevent rotation of the toggle linkage.
Further, U.S. Pat. No. 5,543,595 describes a circuit breaker, which utilizes reversing levers that are attached to a cradle. The reversing levers interact with an upper link and the handle to prevent rotation of the handle to a position where the toggle linkage can rotate if the contacts are welded.
If the weld is of sufficient strength, the contact arm cannot be rotated and the contacts remain closed. Still, it may be possible to rotate the handle to the off position. Furthermore, in some installations, the circuit breaker is operated by a motor operator or other external mechanical means which can force the operating handle to the off position even though the contacts are welded closed. Obviously, this is a very unsatisfactory situation.
Typically a maintenance operator will place the handle of a circuit breaker in the “OFF” position to remove electrical power from the system before doing corrective maintenance. The maintenance operator may also padlock the handle in this position as an added measure to prevent others from placing the breaker in the “ON position while this maintenance is being done.
Thus, a mechanical means is desired to prevent the maintenance operator from placing the handle in the “OFF” or “RESET” position and possibly padlocking the handle in this position, in the event that contacts should become welded and power cannot be interrupted by handle movement. Further, there is a need for an improved positive off mechanism for a circuit breaker which is rigid enough to block movement of the operating handle to the off position despite the application of a force to the operating handle to the “OFF” position when the contacts are welded closed.